Various throttle valve units have been previously proposed. For instance, JP-A-H11-62739 discloses a throttle valve unit connected to the intake pipe 8 of an automobile. This throttle valve unit, as shown FIG. 20 and FIG. 21, includes a main body 9 with a cylinder portion 91 that defines an intake air passage 90, a fastening portion 95 integrally formed on the outer peripheral surface 941 of the cylinder portion 91, and a throttle valve 92 that is rotatably supported by the cylinder portion 91 for opening and closing the intake air passage 90. The fastening portion 95 has a bolt hole 96, and the flange portion 85 of the intake pipe 8 also has a bolt hole 86. A bolt 81 extends through the bolt holes 96, 86 to thereby couple the main body 9 and the intake pipe 8.
Here, as shown in FIG. 20, the fastening portion 95 can shrink and distort when manufactured whereby the bottom surface 97 of the fastening portion 95 becomes cambered. When the cambered bottom surface 97 is fastened to the flange portion 85 of the intake pipe 8 with the bolt 81, as shown in FIG. 21, the bottom surface 97 can be displaced due to the different shape of the abutment surface 87 of the flange portion 85 of the intake pipe 8. This displacement of the fastening portion 95 can be transmitted to the cylinder portion 91 such that the inner peripheral surface 94 of the cylinder portion 91 deforms in the radial direction.
In partial response to this problem, JP-A-H11-62739 discloses a recessed portion 964 formed in a middle portion 963 between the bolt hole 96 and the cylinder portion 91 to reduce the rigidity of the middle portion 963. Since the middle portion 963 is reduced in rigidity, when the fastening portion 95 is fastened to the intake pipe 8, the fastening portion 95 can more easily deflect according to the shape of the abutment surface 87 of the flange portion 85 of the intake pipe 8 due to the fastening force of the bolt 81. Thus, distortion caused when the fastening portion 95 is manufactured is corrected.
Furthermore, JP-A-H8-8229 and JP-A-H10-280981 also disclose a throttle valve unit having a recessed portion formed between a fastening portion and the outer peripheral surface of a cylinder portion.
However, in these prior art devices, a support area 93 is included for supporting the throttle valve 92 in the cylinder portion 91. The support area 93 is spaced along the axis of the cylinder portion 91 from a connecting portion 951 of the fastening portion 95. As such, the support area 93 of the throttle valve 92 is relatively thin and hence lower in rigidity than the connecting portion 951 of the fastening portion 95. For this reason, the support area 93 of the throttle valve 92 can be easily deformed.
For example, the bolt hole 96 is coupled to the cylinder portion 91 via the middle portion 963. Thus, even if the middle portion 963 has a recessed portion 964 and is hence reduced in rigidity as described above, a forcible deforming force acting in the radial direction in the bolt hole 96 when the fastening portion 95 is fastened with the bolt 81 can transmit through the middle portion 963 to the support area 93 of the throttle valve 92.
Moreover, when a hose 88 is fitted on the top end portion 911 of the cylinder portion 91 and is fastened thereto by a hose band 89, the top end portion 911 receives a load acting in the radial direction and hence the inner peripheral surface 94 of the top end portion 911 may be deformed. The deformation of the top end portion 911 can be transmitted to the support area 93 of the throttle valve 92 in the cylinder portion 91.
As such, when the low-rigidity support area 93 of the throttle valve 92 receives the bolt fastening force from the bolt 81 and/or the load by the hose band, the support area 93 can be deformed in the radial direction. When the support area 93 is deformed inside in the radial direction, the inner peripheral surface 94 of the support area 93 in the cylinder portion 91 and the throttle valve 92 may abut, thereby degrading control of the opening of the throttle valve 92.